Nuclear magnetic resonance (NMR) imaging has become an established clinical method for the detection of tissue pathology in the human body. The conventional NMR image is generated by placing a sample in a spatially varying magnetic field and subjecting it to a radiofrequency (RF) electromagnetic signal. Nuclei within the sample absorbs energy from this signal depending upon the type of nucleus, the chemical environment of the nucleus, and the location of the nucleus in the non-uniform magnetic field. The nuclei absorbs and re-emits this energy during transitions between its two spin states.
The frequency of these nuclear transitions is given by the Larmor resonance equation: V.sub.o =B.sub.o /2 where B.sub.o is the magnetic field and is the gyromagnetic ratio, a constant for each type of nucleus. By recording the dissipated energy and correlating this with the location of the nucleus, an image of the sample can be produced.
Although these images primarily reveal anatomical differences, the unique sensitivity of NMR methods to motion has engendered the prospect of their use in the non-invasive assessment of blood flow. The fact that the motion of a fluid through magnetic field gradients has a distinct effect on the phase of the resulting signal was noted soon after the discovery of NMR. With the development of NMR imaging, this physical effect has been used as the basis for several techniques designed to provide information on flow in blood vessels. Current methods use either subtraction of images with different flow sensitivities to remove the static material, or selective excitation of moving material with special pulse sequences.
The method of subtraction of images utilizes additional gradient pulses to augment the signal generated by blood vessels without altering the signal from stationary tissues. By taking the difference between images with and without this augmentation, the stationary portion will cancel out tissue leaving, an image of the vessels alone. See Axel, Leon and Morton, Daniel, "MR Flow Imaging by Velocity-Compensated/Uncompensated Difference Images," Journal of Computer Assisted Tomography, 11(1), 31(1987). The use of imaging pulse sequences can involve the use of an imaging readout gradient in one direction which is unpulsed. See Wedeen, Van J. et al., "Projective MRI Angiography and Quantitative Flow-Volume Densitometry," Magnetic Resonance in Medicine 3, 226(1986). The use of such pulse sequences requires significant RF power to adequately cover the subject's bandwidth over the unpulsed gradient.
The synchronization of NMR imaging sequences to the heart cycle to reduce motion artifacts has been studied. See van Dijk, P. "Direct Cardiac NMR Imaging of Heart Wall and Blood Flow Velocity," Journal of Computer Assisted Tomography 8(3):429(1984). Van Dijk has disclosed a method of generating images of blood flow velocity which are color encoded. However, one velocity image is insufficient to quantify the velocity in more than one direction.